1. Field of the Invention
The invention relates to a method and apparatus for achieving growth-etch deposition of diamond. More particularly, the invention relates to a method and apparatus for controlling the growth-etch cycle during flame deposition of diamond to promote the growth of diamond crystals of superior size and quality.
2. Description of the Related Art
It has been observed in the diamond field that the physical properties of diamond films may be enhanced by varying experimental parameters during film deposition so that growth is periodically interrupted by surface etching. The etch rate of diamond is less than the etch rates of non-diamond carbon deposits. If cycling parameters are properly selected (cycling period, growth period, the etchant gases, the growth gases etc.), it is possible to grow higher quality diamond at higher growth rates.
Various investigators of the artificial growth of diamond have studied the cyclic process of diamond deposition. Cyclic deposition processes involve a growth step alternated with an etching step. Cyclic diamond deposition processes involve a growth step wherein a carbon rich growth mixture is presented at a substrate growth surface followed by an etching step wherein an oxygen and/or hydrogen containing carbonless or carbon-lean gas mixture is presented at the substrate growth surface. For example, in a paper by H. Wang and M. Frenklach entitled Analysis of cyclic deposition of diamond appearing in the J. Appl. Phys., Vol. 70 No. 11, pp. 7132-7136 (1 Dec. 1991), incorporated herein in its entirety and for all purposes, the authors make theoretical predictions about the growth of diamond. Wang et al. state that a cyclic process of diamond deposition wherein two alternating steps, a growth step and an etching step produce better quality diamonds at a higher growth rate than if no etching was conducted. Wang et al. predict, according to their model, that:
{An} increase in the film growth rate, accompanied with the improvement in the film quality, can be achieved by shortening the duration of the total cycle and prolonging its growth part. PA1 {B}est deposition conditions . . . occur at total cycle durations shorter than about 30 ms, with the growth cycle fractions larger than 90%. At these conditions, the predicted sp.sup.2 content {non-diamond} in the film is very low and the average linear film growth rate is about 20 .mu.m/h, i.e. almost ten times larger than the growth rate computed for the original 1+4 min. cycle . . . These results are theoretical predictions. It is now of interest to test them by actual experiment. PA1 No beneficial effects were observed using alternating growth/etch cycles to deposit films. Films grown using CH.sub.4 as the carbon source gas consistently produce higher quality diamond films compared to films grown from C.sub.2 H.sub.4. PA1 Improvement of film quality resulting from increased etching times was not observed for either case. Consequently, there is no benefit in depositing diamond using a cyclic deposit/etch scheme (on the time scale of our experiments) when either hydrogen or oxygen is the etchant. PA1 While the chemistry in a plasma is expected to be different in many respects from that for the thermally driven hot filament or a flame, there are definitely some similarities. PA1 Addition of low levels of O.sub.2 to the reactor gas feed led to accelerated carbon deposition rates and enhanced C.sub.2 H.sub.2 concentrations in the reactor exhaust gas . . . Excessive O.sub.2 additions result in reduced growth rates. We have found no benefit to growing diamond using an alternating growth/etch scheme. PA1 By alternating diamond synthesis with the activation of the diamond surface with oxygen it is shown that good quality diamond can be synthesized at methane concentrations up to 15% in hydrogen. PA1 It is well established that as methane concentration is increased the concentration of non-diamond phases in the diamond increases . . . Alternating chemistry synthesis of diamond has been achieved by alternating between methane-hydrogen discharge and an oxygen-hydrogen discharge . . . Methane and oxygen were alternatively added to the hydrogen discharge to achieve diamond growth (methane cycle) and diamond etching (oxygen cycle) respectively. The concentrations of methane and oxygen as well as the times of the two processes were varied. Typical times, in a cycle, ranged, from 2 to 5 minutes for diamond deposition alternated with 0.5 to 2 minutes for diamond etching. PA1 To achieve a reproducible high quality diamond film by the oxy-acetylene flame method, the acetylene/oxygen flow ration must be controlled very accurately. This is further complicated by the fact that acetone present in the acetylene cylinder influences the oxy-acetylene flame to a varied extent depending on the ambient temperature and the acetylene pressure in the cylinder, i.e. the fraction of acetone included in the acetylene flow varies from experiment to experiment. Although activated charcoal is used in our experimental setup, a slight change in the flame is still present from run to run, presumably due to a small temperature induced drift in the electronics of the mass flow controllers and the residual acetone incorporated into the oxy-acetylene flame. In order to compare several experimental runs, we chose to keep the length of the acetylene feather constant for each deposition by slightly adjusting the electronic mass flow controller set-points.
See id. at pp. 7135. Wang et al. are referring to a cyclic deposition and etching process wherein a total cycle time denoted by t.sub.cycle is the total period of time necessary to complete a singular deposition step followed by a singular etching step. The total cycle time, t.sub.cycle, is the sum of the deposition time denoted by t.sub.growth and the etching time denoted by t.sub.etching. Thus, the total cycle can be expressed as follows: EQU t.sub.cycle =t.sub.growth +t.sub.etching
and the ratio of the deposition time to the cycle time, .tau..sub.growth, is expressed as follows: EQU .tau..sub.growth =t.sub.growth /t.sub.cycle.
At .tau..sub.growth =1.0, the t.sub.cycle =t.sub.growth and t.sub.etching =0. Wang et al. further predict that:
See id. at pp. 7135.
W. A. Weimer, F. M. Cerio and C. E. Johnson examine the chemistry involved in the microwave plasma assisted deposition of diamond. Weimer and colleagues examine the effect of various cycle times and growth times on the quality and growth rate of diamond accomplished by microwave plasma assisted deposition of diamond. W. A. Weimer, F. M. Cerio and C. E. Johnson, authors of Examination of the chemistry involved in microwave plasma assisted chemical vapor deposition of diamond appearing in J. Mater. Res., Vol. 6, No. 10, pp. 2134-2144 (October 1991), incorporated herein by reference in its entirety and for all purposes, state that:
See id. at pp. 2134, 2140. The time scales used by Weimer et al. were t.sub.cycle =4 minutes and t.sub.growth =4 minutes, 3 minutes, 2 minutes and 1 minute, respectively, for the H.sub.2 etch. In addition, the time scales used by Weimer et al. were t.sub.cycle =4 minutes and t.sub.growth =4 minutes, 3.60 minutes, 3.40 minutes and 3 minutes, respectively, for the H.sub.2 /O.sub.2 etch. Weimer et al. do state that
(Emphasis added.) See id. at pp. 2137, 2143.
K. V. Ravi examines the cyclic deposition process of growing diamonds on a substrate using microwave plasma chemical vapor deposition (CVD) using a CH.sub.4 --H.sub.2 growth mixture and O.sub.2 --H.sub.2 etch mixture. In a paper by K. V. Ravi entitled Alternating Chemistry Synthesis of Diamond appearing in Proceedings of the Second International Symposium of Diamond Materials edited by A. J. Purdes, K. E. Spear, B. S. Meyerson, M. Yoder, R. Davis and J. C. Angus, TIIE ELECTROCHEMICAL SOCIETY, Pennington, N.J., pp. 31-38 (1991), incorporated herein by reference in its entirety and for all purposes, the author states that:
See id. at pp. 31. Ravi further states that:
(Emphasis added.) See id. at pp. 32.
Olson and colleagues performed cyclic deposition experiments using a reactor that incorporates a rotating substrate stage and physically separated hot-filament-activated growth and etch sections wherein the substrate stage is rotated through the growth and the etch sections in sequential alternate fashion. In a paper by Olson et al. entitled Sequential Growth of High Quality Diamond Films from Hydrocarbon and Hydrogen Gases appearing in Mat. Res. Soc. Symp., Materials Research Society, Vol. 242, pp. 43-49 (1992), incorporated herein in its entirety and for all purposes, the authors disclose a method of carrying out sequential growth and etching by mounting a substrate on a rotating face plate which is rotated so that the substrate is exposed sequentially to a diamond growth reactant gas mixture followed by exposure to a diamond etchant reactant gas mixture wherein the substrate and reactant gases are heated by hot filament.
In the specific case of diamond film deposition with an oxygen-acetylene torch, the growth-etch cycle may be accomplished by varying the depositing film's exposure to oxygen, a strong etchant of carbon at substrate temperatures typical to diamond deposition (.about.800.degree.-1300.degree. C.). Methods which have been previously utilized to achieve periodic etching of torch-deposited films include (1) cycling of the oxygen supply to the torch nozzle and (2) alternating the position of the depositing film between two different torch flames, one oxygen rich and the other oxygen deficient.
Tzeng and Phillips report on the cyclic deposition of diamond carried out using an oxygen-acetylene torch. Tzeng and colleagues maintained a constant flow of acetylene for combustion in a flame. In addition to the acetylene, excess oxygen was introduced into the flame. The amount of oxygen flowing to the flame tip was introduced in a pulsed (i.e. intermittent) fashion for varying durations of time creating a cyclic process wherein the gas mixture burning in the flame was an oxygen rich (i.e. etch) acetylene mixture alternating with an oxygen poor (i.e. growth) acetylene mixture. The total cycle time, t.sub.cycle, was varied from 50 seconds to 10 minutes and the growth fraction, .tau..sub.growth, was set at 0.8. The paper by Tzeng and Phillips entitled Minimization of Infrared Absorption of Flame Deposited Diamond Films by Sequential Deposition and Etching Processes appearing in PROC. ELECTROCHEM. SOC. 2ND INTERN. SYMP. ON DIAMOND MATERIALS, edited by A. J. Purdes, J. C. Angus, R. F. Davis, B. M. Meyerson, K. E. Spear and M. Yoder, THE ELECTROCHEMICAL SOCIETY, Pennington, N.J., pp. 49-56 (1991), is incorporated herein by reference in its entirety and for all purposes, outlines their process. Tzeng and Phillips note several factors which need to be accurately controlled in order to grow reproducible high quality diamond films:
(Emphasis added.) See id. at pp. 50.
The various methods for carrying out a cyclic growth/etch are reviewed in a paper by Cline et al. entitled Cyclic deposition of diamond: Experimental testing of model predictions appearing in J. Appl. Phys., Vol. 72, No. 12, pp. 5926-5940 (15 Dec. 1992), incorporated herein by reference in its entirety and for all purposes. Cline et al. remark with respect to the Wang et al. reference cited, supra, that the t.sub.cycle and .tau..sub.growth parameters cited by Wang et al. "could not be attained due to equipment limitations." Emphasis added. See Cline et al. at pp. 5927. In fact, none of the papers reviewed have disclosed an apparatus wherein the advantages of a t.sub.cycle =30 ms or less and .tau..sub.growth of 0.9 or greater have been explored.
While it is noted that Mucha et al., Ravi, and Wang et al. utilize a cyclic process for diamond film growth by microwave plasma CVD, there remains a need for a cyclic process for diamond film growth and single crystal diamond growth wherein the parameters of t.sub.cycle and .tau..sub.growth can be manipulated wherein t.sub.cycle is under about 150 ms and .tau..sub.growth is about 0.9 or greater in order to produce higher quality diamond of larger size at a greater growth rate without the flow controller complications encountered. For example, Tzeng et al., supra, have stated that the "acetylene/oxygen must be controlled very accurately." See id. at pp. 50. A simpler apparatus and method needs to be designed to achieve and surpass the results obtained by other investigators thus far. In addition, there is a need for an apparatus and method capable of growing high quality diamond denoted, for example, by a Raman spectrum lacking peaks indicating non-diamond contamination. More particularly, there is a need for an apparatus and method capable of growing high quality diamond which has a Raman spectrum wherein the diamond peak at 1332 cm.sup.-1 has a full width at half maximum (FW HM) between about 3-4 cm.sup.-1. The smaller the value of FWHM the greater the quality and purity of the diamond. Generally, naturally found diamond has a FWHM value of between 2.5-3.0 cm.sup.-1. Generally, synthetic diamond has a FWHM value of about 5.0cm.sup.-1. In addition, there is a need for an apparatus and a method for growing such exemplary high quality diamond at an exemplary rapid growth rate of between about 50-100 .mu.m per hour or greater.